1. Field of the Invention
The present invention relates to a process of treating metal in an acid pickling bath to condition the surface thereof and concerns the use of a novel additive for the acid pickling bath including a compound capable of liberating elemental colloidal sulfur and sodium sulfite and to compositions used in such a process.
2. Description of the Prior Art
The use of a compound capable of liberating colloidal sulfur in an acid pickling bath is well known in the acid pickling bath art and has been used extensively for many years. For example, Irwin discloses the use of sodium thiosulfate or a polysulfide in U.S. Pat. No. 2,636,009. As is generally known, various pickling baths are used to condition the surface of metal including the removal of various oxides, rust or scale and various surface contaminants on the metal. When the metal is so conditioned, subsequent operations such as plating, galvanizing, enameling and painting may be performed on the metal.
In general, acid pickling baths utilize sulfuric acid or hydrochloric acid as well as various combinations thereof. The prior art has recognized that when the acid attacks the undesirable surface on the metal, fissures or crack-like courses in the undesirable surface are developed until the acid reaches the base where the metal itself is more readily attacked. The longer the interval of the bath, the deeper such eating is directly into the metal to etch it away.
Hydrogen gas is generated by the chemical action of the acid on the iron and may be trapped about the crack or fissure to blast off scale particles. The hydrogen gas formed by the pickling acid reacting with the metal has a trait of spreading out or locating as a film coating over the metal and the residual scale or rust on the metal. The absorbed film of gas is too thin to have sufficient buoyancy to rise as a bubble and thus to escape from the bath.
The prior art has recognized the introduction into an acid pickling bath of a compound capable of liberating elemental colloidal sulfur. When the elemental colloidal sulfur is liberated, it reacts with the hydrogen gas to form hydrogen sulfide which escapes from the bath and thereby decreases the amount of the hydrogen film present. When the amount of the hydrogen film present is so decreased, the effectiveness of the pickling acid is increased since the pickling acid can more readily attack the surface of the metal.
The prior art has also recognized other various additional additives for use in conjunction with a compound capable of producing colloidal sulfur. Irwin, U.S. Pat. No. 2,692,187, shows the additional additive of chromium aldonate to act as catho-depolarizer of the couples formed on the surface of the metal which is being pickled. As a result the pickling action is exceedingly rapid and at the same time a uniform attack of the acid on the metal is obtained. In U.S. Pat. No. 2,692,188, an additional additive of a thiocyanate is disclosed to obtain a finely grained etched surface on the metal which is pickled. U.S. Pat. No. 2,831,814 discloses the additional additive a ferrocyanide to provide a whiter pickled product and in U.S. Pat. No. 3,226,180, the additional additive of an organic acid inhibitor is disclosed to decrease the weight loss of the base metal.
All of the systems described above perform satisfactorily to a certain degree and accelerate the rate of acid attack on the metal but it has been found that the present invention provides a system that has improved performance over the systems of the prior art.
Accordingly, it is the primary object of the invention to provide an improved process and additive for an acid pickling bath to condition the surface of metal and remove various oxides, rust or scale and various other surface contaminants on the metal.
Another object of the invention is to provide a process and additive to be used in conjunction with an acid pickling bath which is noted for its exceptional performance characteristics.
Another object of the invention is to provide an improved process and additive for an acid pickling bath whereby it is possible to decrease the temperature of the bath while maintaining the rate of acid attack at the same level as prior art baths which are operated at a higher temperature and thereby save energy.
A still further object of the invention is to provide an improved process and additive for an acid pickling bath which allows for lower acid concentrations, faster line speed and longer tank life.
It is still a further object of the invention to provide an improved process and additive for an acid pickling bath which allows the pickling bath to be effectively used for a longer period of time even though the bath has a higher iron value and lower acid value.
Other objects of the invention will become apparent to those skilled in the art from a reading of the following specification and claims.